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THE AGE OF REPTILES
From one point of view the advance of life on the earth seems to
proceed not with the even flow of a river, but in the successive
waves of an oncoming tide. It is true that we have detected a
continuous advance behind all these rising and receding waves,
yet their occurrence is a fact of some interest, and not a little
speculation has been expended on it. When the great procession of
life first emerges out of the darkness of Archaean times, it
deploys into a spreading world of strange Crustaceans, and we
have the Age of Trilobites. Later there is the Age of Fishes,
then of Cryptogams and Amphibia, and then of Cycads and Reptiles,
and there will afterwards be an Age of Birds and Mammals, and
finally an Age of Man. But there is no ground for mystic
speculation on this circumstance of a group of organisms fording
the earth for a few million years, and then perishing or
dwindling into insignificance. We shall see that a very plain and
substantial process put an end to the Age of the Cycads,
Ammonites, and Reptiles, and we have seen how the earlier
dynasties ended.
The phrase, however, the Age of Reptiles, is a fitting and true
description of the greater part of the Mesozoic Era, which lies,
like a fertile valley, between the Permian and the Chalk
upheavals. From the bleak heights of the Permian period, or--more
probably--from its more sheltered regions, in which they have
lingered with the ferns and cycads, the reptiles spread out over
the earth, as the summer of the Triassic period advances. In the
full warmth and luxuriance of the Jurassic they become the most
singular and powerful army that ever trod the earth. They include
small lizard-like creatures and monsters more than a hundred feet
in length. They swim like whales in the shallow seas; they shrink
into the shell of the giant turtle; they rear themselves on
towering hind limbs, like colossal kangaroos; they even rise into
the air, and fill it with the dragons of the fairy tale. They
spread over the whole earth from Australia to the Arctic circle.
Then the earth seems to grow impatient of their dominance, and
they shrink towards the south, and struggle in a diminished
territory. The colossal monsters and the formidable dragons go
the way of all primitive life, and a ragged regiment of
crocodiles, turtles, and serpents in the tropics, with a swarm of
smaller creatures in the fringes of the warm zone, is all that
remains, by the Tertiary Era, of the world-conquering army of the
Mesozoic reptiles.
They had appeared, as we said, in the Permian period. Probably
they had been developed during the later Carboniferous, since we
find them already branched into three orders, with many
sub-orders, in the Permian. The stimulating and selecting
disturbances which culminated in the Permian revolution had begun
in the Carboniferous. Their origin is not clear, as the
intermediate forms between them and the amphibia are not found.
This is not surprising, if we may suppose that some of the
amphibia had, in the growing struggle, pushed inland, or that, as
the land rose and the waters were drained in certain regions,
they had gradually adopted a purely terrestrial life, as some of
the frogs have since done. In the absence of water their frames
would not be preserved and fossilised. We can, therefore,
understand the gap in the record between the amphibia and the
reptiles. From their structure we gather that they sprang from at
least two different branches of the amphibia. Their remains fall
into two great groups, which are known as the Diapsid and the
Synapsid reptiles. The former seem to be more closely related to
the Microsauria, or small salamander-like amphibia of the
Coal-forest; the latter are nearer to the Labyrinthodonts. It is
not suggested that these were their actual ancestors, but that
they came from the same early amphibian root.
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